This invention relates to an improved method for diffusion coating of surfaces such as chromizing ferritic surfaces and, more particularly, the interior and exterior surfaces of steel boiler tubes, pipes and like components, particularly small bore tubing.
Chromizing is a process used to produce a high chromium surface layer on iron or steel by high temperature heating of a solid packing material containing chromium powder. This process is used on boiler tubes, pipes, and other components, like boiler components, to provide a surface which is resistant to exfoliation, i.e., high temperature oxidation with subsequent breaking away or loss of the oxide layer. Boiler components are often chromized by a process known as pack cementation. This processing technique has been widely used throughout industry for many years.
In the pack cementation process, a pack mixture comprising chromium, an inert filler (e.g., alumina) and a halide activator (e.g., ammonium chloride) are blended together. The boiler component to be treated, i.e., the tubing or pipe, is filled with the mixture. The component is then loaded into a controlled atmosphere retort or sealed by the welding of caps to its ends to produce a self-contained retort. The entire assembly is heated to an elevated temperature and held for a specified length of time to allow the desired chemical reactions and subsequent diffusion process to occur. The high chromium content surface layer is formed on the surface of the component by diffusion of chromium into the iron. The component is then cooled to room temperature. The used pack mixture is removed from the interior. The component is then subjected to a post process cleaning step. The end result of this process is a relatively thick (equal or greater than 0.002 inches) chromium diffusion coating on the internal surface of the tubular boiler component.
This process technique has proven to be effective for chromizing boiler components. However, it has several inherent disadvantages. For example, the mix preparation, loading, and removal steps are tedious and time consuming. The gravity loading techniques, which are typically employed for filling elongated tubular components, require shop areas with high ceilings or floor pits, or both, to accommodate components as long as 30 feet in length.
In addition, it is difficult to control pack mix density and composition along the length of the small bore of tubular components (e.g., less than one inch internal diameter) with normal gravity filling techniques. Mix removal and post process cleaning can also be a problem in small bore tubes. Moreover, diffusion thermal cycles are relatively long due to the poor thermal conductivity of the pack mix. Finally, large quantities of pack mix can be required since the internal cavity of the component to be chromized must be filled, and this is quite expensive.
Therefore, a need exists for an improved method of diffusion coating particularly as relates to chromizing of boiler tubing. Moreover, a general technique for chromizing as well as applying diffusion coatings of other elements, for example, silicon, aluminum and boron, to various configurations and shapes would have significant advantages and widespread application.